1. Field of the Invention
The present invention relates in general to a displacement information detector. In particular, the invention relates to a displacement information detector which is suitable for industrial measuring machines and the like, and which is capable of obtaining a physical amount of displacement information such as position information, rotation information or movement of a displaced object with high accuracy by utilizing the diffraction caused when the displaced object (optical scale) is irradiated with light.
2. Related Background Art
Heretofore, in order to detect displacement information such as position information, an amount of movement or an amount of rotation of an object (displaced object), many displacement information detectors (encoders) such as rotary encoders or linear encoders are used in industrial measuring machines and the like.
The applicant of the present invention has proposed the various encoders of the so-called grating interference system for detecting the fluctuation in position or velocity of an object by applying the diffraction interference phenomenon of light until now. In particular, the applicant of the present invention has proposed the encoder in which a fine scale of micron order is adopted, and two luminous fluxes diffracted by the fine scale are taken out to be made interfere each other to thereby obtain the much higher resolution than that of the encoder of a geometrical optics system.
These encoders adopt the construction in which the wave surfaces of two diffracted rays of light are composed to produce the interference pattern. However, since the encoders are of an interference optical system, the very strict accuracy is required for the processing and the arrangement of optical elements. In particular, in the case of the so-called embedded type encoder in which a scale portion and a detection head portion are separated from each other, a user must fit the scale and the detection head portion to a motor, a stage or the like, and hence the difficulty in assembly in the work thereof becomes a problem. In addition, in the case where such an encoder is fitted to the actual apparatus, a smaller encoder has been required along with the miniaturization of the apparatus itself.
Then, until now, the applicant of the present invention has proposed the encoder which is adapted to detect the highly accurate displacement information and which reduces the influence of the alignment error during the installation by utilizing a correction optical system adapted to correct the errors in assembly of various optical elements in Japanese Patent Application No. 2001-25124 for example.
FIG. 4 is a schematic view showing construction of a main portion of an optical system of an encoder which can detect displacement information with high accuracy by utilizing the correction optical system which the present applicant previously proposed.
In FIG. 4, a luminous flux R emitted from a semiconductor laser LD permeates through a partial transmission portion W of a beam splitter BS to be applied to a diffraction grating scale (scale grating) GT through a reflecting mirror M1 and a transmission portion of a toric (circular ring) element CG. The reflected and diffracted rays of light R+ and R− diffracted in the scale GT are applied to toric reflection gratings CG1 and CG2 of the toric element CG, respectively. Here, assuming that the grating pitch on the diffraction grating scale GT is P1, the grating pitch P2 of the toric reflection gratings CG1 and CG2 is set so as to meet the following relationship.P2=P1/2
The toric reflection gratings CG1 and CG2 operate as the diffraction grating having the grating pitch P2 when viewed locally. Then, the luminous fluxes are diffracted to the original azimuth (on the diffraction grating scale GT side) to be applied to nearly the same position of the diffraction grating scale grating GT to be rediffracted, and then the luminous fluxes are combined with each other to trace the original path to be returned back to the beam splitter BS. The luminous fluxes are taken out in the direction different from the semiconductor laser LD in the reflection diffraction grating GT4 of the rear face of the beam splitter BS to be detected as the interference flux by a light receiving element PD4. By the way, in the case where ± primary diffracted rays of light are used, the light and darkness periods of the interference on the light receiving element PD correspond to four periods per movement for one pitch of the diffraction grating scale grating GT.
The encoder in this prior art example has the effect of correcting the optical path shift for the wavelength fluctuation of the light source due to the effect of the toric reflection gratings CG1 and CG2. Since the correction is also exerted on the alignment errors of the optical elements, even in the case of the encoder in which the scale grating GT and the detection head (PD4) are separated from each other, the installation thereof becomes relatively easy. In addition, since the number of constituent components or parts is very small, the miniaturization and thinness thereof become possible.
Assuming that the grating pitch on the diffraction grating scale GT is P1 in the encoder shown in FIG. 4, the pitch P2 of the toric reflection gratings CG1 and CG2 is set so as to meet the following relationship:P2=P1/2.
Thus, in particular, when the diameter of the disk scale of the rotary encoder is made small, if the luminous flux illumination position is radially shifted, there is encountered a problem in that the pitch P2 becomes easy to be shifted from the setting.
Then, in particular, when a disk with a small diameter is used, there is desired a three grating interference optical system in which the stable displacement information independent of the shift of the flux illumination position in a radial direction is obtained.